1. Field of the Invention
The present invention relates to an optical multiplexer/demultiplexer to be used for an optical multiplexer/demultiplexer, etc. of an optical communication device and manufacturing method thereof.
2. Description of the Related Art
Conventionally, in optical multiplexers/demultiplexers of this kind, research has been carried out on a dielectric-filter type multiplexer/demultiplexer having a multi-layer structure in which a plurality of half-mirror type dielectric-filters are disposed, a multiplexer/demultiplexer having a structure called FBG (Fiber Bragg Grating) constituted by connecting many optical fibers with a grating structure, a multiplexer/demultiplexer having a bulk diffraction grating structure constituted by connecting a bulk-state diffraction grating to a plurality of optical fibers, and a structure called AWD (Arrayed Waveguide Grating) which is constituted by incorporating a light waveguide structure in the middle of an optical fiber.
However, there has been a problem in that optical multiplexers/demultiplexers of dielectric-filter type have a limitation of achieving multi-channel signal, and thus the cost increases with the increase of the number of channels. Also, in optical multiplexers/demultiplexers with the FBG structure, there has been a problem in that a high-cost part called circulator becomes necessary. Further, in the structure in which bulk diffraction grating is connected, there has been a problem in that the reliability in optical connection might cause a problem, and that assembling elements is difficult. Furthermore, in the case of the AWD structure, there has been a problem in that an insertion loss is large, temperature dependency might arise in the elements, thus temperature control is necessary, and a peltier device is necessary for cooling.
From the above background, various researches and developments are underway in optical multiplexers/demultiplexers. However, the above dielectric-filter type, the FBG structure type, and the AWD structure type have difficult problems to solve in that they are high cost and an insertion loss is big. Thus the present inventor focuses attention on an optical multiplexer/demultiplexer using a diffraction grating.
FIG. 24 shows an example of a well-known demultiplexer using the conventional diffraction grating of this kind. The optical demultiplexer 121 consists of an input optical fiber 100, output optical fibers 101, 102, and 103, first lenses 105, 106, 107, and 108, semiconductor lasers 110, 111, 112, and 113, second lenses 115, 116, 117, and 118, and a diffraction grating 120.
In the optical demultiplexer 121 shown in FIG. 24, the above-described input optical fiber 100 and the output optical fibers 101, 102, and 103 are disposed in alignment, and an outgoing light direction of the output optical fibers 101, 102, and 103 is 180° opposite direction to an incident light direction of the input optical fiber 100.
The first lens 105 and the semiconductor laser 110 are disposed in front of the input optical fiber 100. Also, the first lens 106, the semiconductor laser 111, and the second lens 116 are disposed at the back of the outgoing light direction of the output optical fiber 101 in sequence. The first lens 107, the semiconductor laser 112, and the second lens 117 are disposed at the back of the outgoing light direction of the output optical fiber 102 in sequence. The first lens 108, the semiconductor laser 113, and the second lens 118 are disposed at the back of the outgoing light direction of the output optical fiber 103 in sequence. Then the diffraction grating 120 is disposed facing the second lenses 115 to 118.
The operation of the optical demultiplexer 121 having a structure shown in FIG. 24 will be described. First, light including the light having different three wavelengths λ1, λ2, and λ3 enters from the input optical fiber 100 into the semiconductor laser 110 through the first lens 105. The incident light is amplified by the semiconductor laser 110, and enters the diffraction grating 120 through the second lens 115. Light is reflected at a different angle for each wavelength by the diffraction grating 120. Thus each of the light having wavelengths λ1, λ2, λ3 is amplified by each of the corresponding semiconductor lasers 111, 112, and 113 through each of the second lenses 116, 117, and 118, respectively, passes through each of the first lenses 106, 107, and 108, and is output from each of the output optical fibers 101, 102, and 103.
The optical demultiplexer 121 having a structure shown in FIG. 24 can accommodate multiple channels, and has little limitation of cost increase because expensive parts, such as a circulator, are unnecessary. However, there has been a problem in that assembling component elements is difficult, and thus miniaturization is difficult. Specifically, a light path from the input optical fiber 100 to the diffraction grating 120 through the first lens 105 and the second lens 115, and light paths from the diffraction grating 120 to the output optical fibers 101, 102, and 103 through the second lenses 116, 117, and 118, and the first Lenses 106, 107, and 108, respectively need to be positioned exactly. Thus an assembling process including positioning each of the parts constituting the demultiplexer tends to become extremely complicated, and the demultiplexer has a structure which is difficult for mass production.